Greg_S. • Adobe Community Professional, Nov 08, 2020
Do you remember what you were trying to do when the toolbox went to one column?
I have no idea whether these suggestions will work, but try the following:
Press the Tab key to toggle the toolbox on and off
Reset all tools from the toolbin
Try resetting the preferences file. Go to Edit>Preferences>General tab and click the Reset Preferences on next launch button.